This invention relates to a method for shaping a material with a reticular structure, or a net structure, which also includes a plant for performing same and the reticular material resulting from the method.
The method in question is applied to handling a synthetic plastics material with a reticular structure, in particular one having a tubular shape, which is one of the type with a known constitution and obtained by means of an extrusion process.
The reticular material resulting from the said method is particularly applied to the packaging industry in general, and it is worth mentioning how important its use is in packaging food products and, more specifically, fruit and vegetable products.
For many years the use of net containers has been known, either of the knotted type, such as net bags, or of the type bound by plain knitting, such as the bags of a loosely woven fabric, like hemp canvas, sackcloth, matting, leno, etc. to transport onions, potatoes, etc., on a commercial level, or of the type obtained by extruding a synthetic plastics material, such as the one used to make bags for packaging various products used on a domestic level, such as oranges, potatoes, onions, nuts, chestnuts, lemons, etc.
The Spanish Utility Model No. 158.340 is known, which describes an extruded net that is mono- or biaxially oriented, without this transcending to the cross points, which has low tensile strength, good dimensional stability and reduced coverage ability.
The actual applicant is the owner of Spanish Utility Model Nos. 290.122, 290.123, 290.124, 290.125 and 290.126 that relate to different variants of tubular flattened extruded net arrangements, made up of two or three non-molecularly oriented filaments, having the characteristics of low tensile strength, good dimensional stability and high coverage ability.
Patent EP 0 788 974 is also known, which relates to a packaging made up of non-oriented extruded net which is folded with a plastic film and joined to it by welding, and Patent ES 2.014.712 which describes packaging bags made from woven net with plastic filaments which is folded with a plastic film joined to the woven net by welding.
Moreover other patents are known that are related to this application, and they are:
U.S. Pat. No. 3,140,330 relates to a system for drawing, longitudinally and transversally, a tubular net after it has been extruded controlling its drawing with a calibration ring and a hot bath, with the net being dragged by a winder, not shown, and guided by two lower submerged rollers, which do not come into contact with one another, and a top guide roller. In this patent no operation for laminating the tubular flattened net resulting from the drawing operation, is envisaged.
U.S. Pat. No. 2,002,711, relating to a method for producing a flat, continuous and smooth web of cellulosic material, in other words, it is not a reticular tubular structure, although it envisages using calendering rollers to surface the web of material. Nevertheless, the plant shown in said patent to describe the process for producing the flat, continuous and smooth web is slightly reminiscent of possible, non-claimed variants described in this invention and which correspond to FIGS. 13 and 14 in the description.
U.S. Pat. No. 3,968,621 relates to a method for obtaining a flat device for transporting cans of drink, which is made up of a plurality of cables which, describing irregular trajectories on a plane that determine meetings between pairs of said cables, form a flat grid with openings slightly smaller than the section of the cans to be contained. This narrow web which is flat, and therefore not tubular, and is not molecularly oriented by the corresponding longitudinal drawing and transversal widening operations, is a different starting material to that which is the object of this patent application, and it is not to be understood that this is affected by the fact that in some case said narrow support web for drink cans undergo a calendering operation, which is performed by flattening the cables and intersection points until they are reduced to a perforated lamina that is similar to the equivalent narrow webs obtained through die-cutting or punching the laminar material.
FR patent 1.566.717 relates to a plastic mesh net intended to cover boxes for transporting fruit and vegetables, which is obtained from a plastics material that is shaped by extruding a material with a reticular tubular structure that is subjected to a single longitudinal drawing operation, without being accompanied at any time by a transversal widening operation, after which the material with a reticular tubular structure opens, transformed into a laminar material such as those in FIG. 1 and A of FIG. 5, with the material in the above-mentioned shape being subjected to a calendering operation to give it a flat shape.
U.S. Pat. No. 3,370,116 relates to a method for widening a reticular tubular structure up to a predetermined size, this aim being achieved with a mandrel 12, the projection of a hot fluid in station A and a cooling operation in station B, feeding the material by dragging it on rollers 18 without this in any way suggesting a laminating effect, since said rollers 18 do not form a laminating calendering machine.
Patent EP 0 067 238 relates to a method and apparatus for the continuous biaxial orientation of an extruded web of plastics material that is dragged and surfaced by passing through the two rollers 16A and 16B to obtain the longitudinal drawing thereof, which is followed immediately by a widening operation performed by the widening blades 18, without this at any time producing a laminating operation of the web, which undoubtedly does not have a reticular tubular structure.
Finally, Patent ES 2.023.601 is known, which describes a method for obtaining an elastic net for carrying loads, as well as the net obtained therewith.
Moreover, this section must also consider the application field of the material that is the object of the invention, the use of which can be limited, on the one hand, to packaging food products in general and fruit and vegetables in particular and, on the other hand, to containing loads stored on pallets.
With respect to the case of the first application, it is worth highlighting, owing to their extensive use and good functional suitability, the packagings made of reticular net and auxiliary printed elements such as bands of printed film, paper labels, plastic labels, etc.
In those packagings that include net and plastic bands, the net may be of the woven, oriented extruded, or non-oriented extruded type. Each of these types of net has, for its main function as the element for containing the product inside the packaging, certain advantages and disadvantages, namely:
Woven net has a good strength-weight ratio, which makes it possible to optimise the cost of the packaging. It also has the advantage that, because of its light weight and volume, the spools can handle considerable meters of material at one time, which increases the work autonomy of automatic packaging machines. The disadvantage of woven net is that it has very low dimensional stability, and so the packagings formed using this type of net are easily deformed, which has a negative effect on the vertical structure and the ability to display the message or design printed on the packaging.
The packagings obtained with oriented extruded nets have the same advantages and disadvantages as those made from woven net, with the added fact that, generally, oriented extruded nets offer a low product coverage factor because the oriented threads have a small section.
From non-oriented extruded nets, packagings that maintain their vertical structure and provide a clear view of the design printed on the film bands are obtained. Nevertheless, these nets have a non-too favourable strength-weight ratio, and so heavyweight net has to be used, which negatively affects the cost of the packaging. On the other hand, since its threads are not oriented and they have a substantial section, these nets have a high product coverage factor, but at the same time, the spools can only handle a few meters at a time, which causes frequent stoppages in the automatic packaging machines to replace the empty spools.
With respect to the second case of application, reticular structures or flat nets are used when it is essential that air circulates inside the palletised load, so as not to produce water vapour, as is the case of pallets containing boxes of fruit, vegetables or fresh vegetables; or when hot elements are arranged which are to cool in the atmosphere, such as bricks and ceramic pieces just after they have been fired.
These contention elements can be perforated films, woven nets or extruded bi-oriented nets.
Perforated films have good elasticity and they are not very thick, and so a high number of meters can be provided on one spool, and they do not produce loose threads or a large amount of waste when the pallets are undone. As for the disadvantages, it is worth mentioning that they have limited tensile strength and that, since to maintain their cohesion the perforated surface cannot be very high, the ventilation effect inside the palletised load is very much reduced.
Extruded nets offer good tensile strength, but they suffer from the drawback that, owing to their thickness and rigidity, only a few meters fit on the spool and that the net takes up considerable space when the pallets are undone, and it has to be placed in waste containers.
Woven nets do not offer as good a shearing strength as the extruded oriented ones, particularly when the palletised load has ends with edges, but their main drawback is that when the net is cut to undo the pallets, threads come off that accumulate in the wheels and axes of the back pallet parts and stacking trucks, and that can cause breakdowns or, it at least means that the cleaning and maintenance frequency of these machines has to be increased.